1. Field of the Invention
The present invention relates to an optical waveguide sensor using an evanescent wave that leaks from a core of an optical waveguide. The present invention also relates to a manufacturing method of an optical waveguide sensor.
2. Description of the Related Art
JP-A-2005-61904 discloses an optical waveguide sensor using an evanescent wave that leaks from a core of an optical waveguide.
The optical waveguide sensor is made of a silicon-on-insulator (SOI) substrate. The optical waveguide includes a silicon thin-line core and a clad layer. The silicon thin-line core is formed by processing a silicon layer located on a buried oxide layer in the SOI substrate. The clad layer is made of silicon oxide and fills either ends of the silicon thin-line core.
An upper surface of the silicon thin-line core is exposed to an outside in a detection region. In the detection region, the silicon thin-line core is shuttles at predetermined intervals, that is, the silicon thin-line core is arranged in a meandering shape.
In the above-described optical waveguide sensor, a length of the silicon thin-line core can be secured while restricting a dimension by arranging the silicon thin-line core in the meandering shape, and thereby a detection sensitivity of the optical waveguide sensor is improved. In a case where the silicon thin-line core is arranged in the meandering shape, it is required to reduce a bending radius at a bent portion where a waveguide direction is changed in order to make a small and long waveguide.
When the bending radius is small, at the bent portion, an incident angle may be smaller than a critical angle and light may be easily transmitted from the core. Thus, in the above-described waveguide sensor, a difference in refractive index between the core made of single crystal silicon and the clad made of silicon oxide is increased so that a reflection angle is reduced and light is not easily transmitted from the core even at the bent portion.
However, when the reflection angle is reduced by increasing the difference in refractive index between the core and the clad, a loss generated at an interface due to scattering may be increased. Thus, the above-described optical waveguide sensor may have a large transmission loss and may have difficulty providing a small and long optical waveguide.
If the difference in refractive index between the core and the clad is reduced in the above-described configuration so as to increase the bending radius at the bent portion, the transmission loss can be reduced. However, because the silicon thin-line core is arranged in the meandering shape, increase in the dimension of the silicon thin-line core at each bent portion greatly influence the dimension of the optical waveguide sensor, and the dimension of the optical waveguide sensor is increased.